md: dm-req-crypt: Initialize global variables in constructor

dm-req-crypt was not initializing number of engines available for
crypto operation. Hence, number of engines were getting accumulated
everytime a device based of dm-req-crypt was created. This caused
crash in qcrypto module while retrieving the crypto engine.

Change-Id: I06b5b296a80ae4f9f6bfd024222be9f47a29bfce
Signed-off-by: Dinesh K Garg <dineshg@codeaurora.org>